Claiming that closeted gay men have a duty to reveal their sexuality for the greater good of society means that you think you have a right to that information.
And, newintown, I agree with you that straight celebrities do not have to keep their sexualities hidden. No one asks straight people to keep quiet about who they are. But you're implying that queer people stay in the closet because they are ashamed of their sexuality. While that is certainly the case for some people, who have been conditioned so deeply by society to hate who they are so that they cannot live their true lives, for the most part queer people remain in the closet because, as it stands right now, it is not safe to be queer in America. Marriage equality did not suddenly fix the rampant homophobia woven into American society. Still, every single day, queer people are harassed, abused, and murdered because of their sexuality. And while it is unlikely that Kevin Spacey, assuming he is queer in some way, would face this level of antagonism as a celebrity of his stature, his career most certainly could be affected. I also wouldn't blame him for simply wanting to avoid the vitriolic hate that would surely come his way by the thousands.
The responsibility is on the general public, especially straight people, to dismantle heteronormativity, until we stop considering that straight is the standard and everyone else is deviating from the norm. If he is queer, it is certainly not on Kevin Spacey's shoulders. I applaud any celebrity who comes out for their bravery in the face of the focused homophobic attacks they are sure to receive, and for helping the fight to de-stigmatize queer people's existence, but I do not feel that anyone in the closet has a duty to do so. And, closeted actors coming out is not the only way to generate queer visibility in the public eye; supporting out and proud performers so that they become celebrities works, too. Supporting queer directors and screenwriters so that they can tell their stories, or are more open to hiring queer actors, does as well.